Gas shrouded wave improvement

ABSTRACT

A cover plate extends over at least part of a solder reservoir and has a slot for a solder wave to extend up above the cover plate. Shield gas is supplied under the cover plate to blanket the solder wave and prevent air from contacting the solder wave and circuit boards passing through the solder wave. Circuit boards are generally supported by conveyor fingers at side edges and the fingers pass through the solder wave. End shrouds are provided at both ends of the solder wave to prevent the entry of air through the conveyor fingers and to ensure that the side edges of the circuit board are uniformly blanketed by shield gas directed to the side edges passing up through the slot in the cover plate. The under surface, front edge, back edge and side edges of a circuit board are therefore uniformly blanketed by shield gas as the board passes through a solder wave.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part of co-pendingapplication Ser. No. 07/961,781 filed Oct. 15, 1992, now U.S. Pat. No.5,240,169, which in turn is a continuation-in-part of co-pendingapplication Ser. No. 07/860,316 filed Mar. 30, 1992, now U.S. Pat. No.5,203,489, which in turn is a continuation-in-part of co-pendingapplication Ser. No. 07/804,904 filed on Dec. 6, 1991 now abandoned.

TECHNICAL FIELD

The present invention relates to wave soldering of elements such asprinted wiring boards and circuit boards, and more specifically tosolder coating of wettable metallized surfaces or solder joining atleast two wettable metallized surfaces on wiring boards in a solder wavewherein the wave is blanketed in a shield gas atmosphere and whereinboard ends are also blanketed with shield gas prior to, during andsubsequent to the passing of the board through the solder wave.

BACKGROUND ART

Today printed wiring boards and circuit boards have smaller wettablesurfaces to be solder coated and joined. In co-pending application Ser.Nos. 07/961,781 and 07/860,316 is disclosed gas shrouded wave solderingwherein a solder wave has a cover with a slot for the solder wave toproject therethrough and wherein a shield gas, preferably nitrogen andsubstantially oxygen free, is supplied underneath the cover so that thesolder is applied to circuit boards and the like when they aresubstantially blanketed by the shield gas. In the aforesaid co-pendingapplications, soldering occurs in a shield gas atmosphere, referred toas an inert gas and this includes nitrogen. The shield gas preventsoxides forming on the liquid solder surfaces, and in another embodiment,a reducing gas such as hydrogen is included in the shield gas in anon-explosive mixture, to provide fluxing to the surfaces to be solderedor solder coated. U.S. Pat. No. 5,044,542 discloses shield gas wavesoldering wherein shield gas or a reducing gas blankets the solder waveduring the soldering step. The benefits of soldering in an atmospherewhich is substantially oxygen free are known.

In our co-pending applications a cover is provided to at least partiallycover a solder reservoir and shield gas passing on both sides of thesolder wave blankets the solder wave and the solder within thereservoir. By utilizing a cover one is able to reduce the flow of shieldgas for blanketing the solder wave and the circuit board or wiring boardpassing therethrough compared with a tunnel or enclosure. Thus, wettablesurfaces to be solder coated such as circuit tracks, pads, componentleads, surface mounted components and metallized holes in boards withpins therein are kept substantially free of air. The shield gas which ispreferably nitrogen and is substantially oxygen free, is introducedbelow the cover and passes up through a slot through which the solderwave projects on both sides of the solder wave to blanket the solderwave and also blanket the bottom surface of a circuit board passingthrough the solder wave as well as the edges of the circuit board.

Circuit boards are conveyed to pass through a solder wave by differenttypes of conveyors. In one embodiment the circuit boards are supportedby gripping fingers from two separate parallel conveyors. In anotherembodiment the circuit boards are supported on pallets, fixtures orframes which in turn are supported either by two separate conveyorsgripping side edges, or by a single conveyor. If two parallel conveyorsare used with gripping fingers, then there is provision to adjust thedistance between the conveyors to take into account variable widths ofconveyors or different sizes of pallets, fixtures or frames to supportcircuit boards.

DISCLOSURE OF INVENTION

When shield gas passes up through the slot in the cover plate on bothsides of the solder wave it blankets the under surface of the leading orfront edge, and the trailing or rear edge of the circuit board, but, theblanketing is not always uniform and the side edges of the circuit boardin some instances are only partially blanketed as air, perhaps entrainedin the conveyor fingers dilutes the shield gas at the ends of thecircuit boards. Thus, it is an aim of the present invention to provideend shrouds in the form of panels that direct shield gas passing upthrough the slot in the cover on both sides of the solder wave to alsoblanket the side edges of the circuit board and preferably ensure thatthere is a substantially uniform blanketing of the circuit board on theunder surface, the front edge, rear edge and side edges prior to, duringand subsequent to the passing of the circuit board through the solderwave.

By utilizing end shrouds it has been found that the consumption ofshield gas required to blanket the under surface and edges of the boarduniformly is reduced considerably and this results in one being able toprovide a shield gas, in one embodiment, nitrogen produced by a membranesource. The nitrogen, while being substantially oxygen free, may in somesituations have up to 10% by volume present in the shield gas.

The present invention provides a process of wave soldering an elementcomprising the steps of projecting at least one solder wave from asolder nozzle above a solder reservoir containing solder, through a slotin a cover means over at least a portion of the solder reservoir;providing gas delivery means positioned on each side of the solder waveunderneath the cover means to pass shield gas upwards through the sloton both sides of the solder wave and blanket the solder wave; supportingthe element by conveyor means; conveying the element by the conveyormeans in a predetermined path above the cover means so that at least aportion of the element passes through the solder wave while blanketed bythe shield gas, and directing shield gas passing upwards through theslot to blanket side edges of the element during the passing of at leasta portion of the element through the solder wave.

In another embodiment there is provided an apparatus for wave solderingan element comprising a solder reservoir adapted to contain moltensolder, and having at least one solder wave nozzle projecting therefrom;pump means for forming a solder wave from the nozzle; cover means forcovering at least a portion of the reservoir having at least onelongitudinal slot for the solder wave to pass therethrough; gas deliverymeans for supplying gas underneath the cover means, the gas passingupwards through the slot on both sides of the solder wave to provide agas blanket over the solder wave; conveyor means for supporting theelement and moving the element in a predetermined path over the covermeans ensuring at least a portion of the element passes through thesolder wave, and end shroud means over each end of the solder wave todirect the gas passing upwards through the slot in the cover means toblanket side edges of the element during the passing of at least aportion of the element through the solder wave.

BRIEF DESCRIPTION OF DRAWINGS

In drawings which illustrate embodiments of the present invention,

FIG. 1 is a side view showing a solder wave with a cover plate and endshroud plate and having gripping fringes of a conveyor supporting acircuit board,

FIG. 2 is a longitudinal sectional view taken at line 2--2 of FIG. 1,

FIG. 3 is a partial longitudinal sectional view similar to FIG. 2,showing the gripping fingers of the adjustable conveyor repositioned togrip a smaller circuit board,

FIG. 4 is a sectional view at line 4--4 of FIG. 2,

FIG. 5 is an isometric view showing the cover plates and end shroudplates for a solder wave according to one embodiment of the presentinvention and also showing a nitrogen supply utilizing membranetechnology.

MODES FOR CARRYING OUT THE INVENTION

As shown in FIG. 1 a solder reservoir 18 has solder therein up to asolder level 20. A solder wave 22 projects from a solder nozzle 24 and asolder pump 26 pumps solder up through the nozzle 24 to form the solderwave 22. The solder pump 26 is preferably a variable speed pump, thusone is able to control the flow of solder from the solder wave 22 andraise or lower the crest of the solder wave 22 to suit specificsoldering conditions.

A cover plate 28 extends over a portion of the reservoir 18 on bothsides of the solder wave 22 and has a slot 30 therebetween through whichthe solder wave 22 projects. The solder wave 22 shown is abi-directional non-turbulent solder wave and has guides 32 to provide asmooth streamline flow of solder falling back into the solder reservoir.The present invention is not limited to any one type of solder wave.Skirts 34 extend downwards from the cover plate 28 beneath the solderlevel 20 to form an enclosure around the solder wave 22 except for theslot 30. Gas diffusers 36 are positioned under the cover plate 28 onboth sides of the solder wave 22 and shield gas is supplied through thediffusers to flow upwards passing through the slot 30 on both sides ofthe solder wave 22 and blanket the solder wave.

As seen more clearly in FIG. 2, there are two cover plates 28 whose sideedges fit in grooves 37 in the opposing side skirts 34. The cover plates28 may be moved in the grooves 37 and clamped in place so the width ofslot 30 can be adjusted to suit different configurations of solder wave22. In the embodiment shown, the side skirts 34 are integral with thesides of the nozzle 24 so the cover plates 38 are supported from thesolder nozzle 24. In another embodiment the side skirts 34 are adjacentto sides of the nozzle 24 and provision is made for varying the heightof the cover plates 28 relative to the solder nozzle 24.

The cover plates 28 are sloped upwards parallel to a predetermined path38 of two conveyors 39,40 which have gripping fingers 41,42 respectivelyto support side edges of a circuit board 44, as seen more clearly inFIG. 2. Whereas the predetermined path 38 is shown sloped, in anotherembodiment the path 38 may be substantially horizontal.

As shown in FIG. 2, skirts 34 extend down from the cover plate 28 on allfour sides to ensure that the solder wave 22 is completely enclosed bythe skirts 34, thus shield gas supplied through diffusers 36 preventsair from contacting the solder wave or the reservoir solder surfaceunder the cover plate 28. The first conveyor 39 is supported from anadjustable rail 46 and has a ramp 48 for the returning conveyor fingers41, the ramp 48 raises the returning fingers 41 so they are out of theway for an end shroud panel 50 which has silicone rubber V-shaped ribs52 in the form of a U, the side ribs rest on top of the cover plate 28on both sides of slot 30 projecting almost as far as the fingers 39 andthe joining rib passes through a tip of the solder wave 22 as can beseen in FIG. 4, thus substantially preventing shield gas exiting fromthe end of the solder wave 22 as it passes upwards through the slot 30in the cover plate 28. The end shroud panel 50 has a vertical plate 54to form a right angle with the end shroud panel 50 and has slots 56therein as shown in FIG. 5 for bolts 58 to attach to an angle bracket 60fixed to the adjustable rail 46 supporting the first conveyor 39. Thus,when the adjustable rail 46 is moved to another position for thegripping fingers 40 to support different sized circuit boards 44, thenthe end shroud panel 50 and ribs 52 move with it to ensure that thegripping fingers 41 and side edges of the circuit board 44 are alwaysblanketed by shield gas passing up through the slot 30 in the coverplate 28. The second conveyor 40 is attached to a fixed rail 62 and anend shroud panel 64 with V-shaped ribs 66 of silicone rubber ispositioned underneath the fixed rail 62 resting on the cover plate 28.

As shown in FIG. 3, when the adjustable rail 46 supporting the firstconveyor 39 is moved across the solder wave 22 to support a smallercircuit board 44, then an additional end shroud panel 68 having V-shapedribs 70 of silicone rubber is placed between the end shroud panel 50 andthe end of the solder wave 22 shown at skirt 34 in FIG. 3. Thus, thesolder wave 22 is not open to air. Shield gas passing up through slot 30in the cover plate 28 covers the solder wave and does not escape intothe atmosphere. In this manner the flow of shield gas is reduced and atthe same time the shield gas blankets the opposite side edges of thesolder wave and consequently blankets the side edges of the circuitboards 44 as well as the underside, and front edge and rear edge, priorto entering the solder wave, during the time that the circuit boards 44pass through the solder wave and after they have left the solder wave.The solder wave replaces the gas blanket as a board passes through.

Whereas the drawings show conveyor fingers 41,42 gripping a circuitboard 44, in other embodiments, the boards are supported on pallets,fixtures or frames, in some cases specially made for different types ofelements. The conveyors may have fingers to grip side edges oralternatively may be supported from above. In all cases the side edgesof the elements, such as circuit boards are blanketed during theirpassage through the solder wave.

A vibrating vane 72 is connected to a vibrator 74 by rod 76 to provide avibration to the solder wave. The solder wave is oscillated inaccordance with the vibratory wave soldering arrangement as disclosed inU.S. Pat. No. 4,684,056.

An in situ shield gas supply source is provided in one embodiment. Thisis feasible because the gas supply is less than required for existingshield gas soldering systems. Nitrogen, carbon dioxide and other inertgases may be supplied as a shield gas. An on site non-cryogenic nitrogensupply may be provided. There are different devices known to producenitrogen with less than 10% by volume oxygen present.

A nitrogen supply source is shown schematically in FIG. 5 whereincompressed air is supplied through a filter 80 and into a membraneapparatus 82, the membrane apparatus includes a membrane therein whichsplits the air flow into two portions, one being a nitrogen enrichedportion and this portion continues along line 84, the oxygen enrichedflow portion passes through line 85 back into the atmosphere. In oneembodiment the membrane apparatus produces nitrogen with an oxygencontent of less than 10% by volume. In another embodiment not shownherein a second stage membrane system may be provided wherein the oxygencontent of the nitrogen is reduced to less than 5% by volume and inanother embodiment to less than 1,000 ppm. The system for providing anitrogen enriched air flow using the membrane technology is known. Ifthere is a high demand of nitrogen, and this depends primarily uponsoldering conditions, then the concentration of oxygen is higher,however, if the demand is less, then the concentration of oxygen isless. In one embodiment the requirement of nitrogen passing through twocontrol valves 86 which in turn are connected to the gas diffusers 36 oneach side of the solder wave 22 beneath the cover plate 20, is in therange of 200 to 600 cubic feet per hour from both diffusers which isconsiderably less than normally required for shield gas soldering. Ascan be seen, the gas supply to the two diffusers 36 can be varied by theflow control valves 86 which in turn controls the flow of gas exitingfrom the slot 30 in the cover plate 28 on each side of the solder wave22. A preferred process provides for the nitrogen supply to be turnedoff except when a printed circuit board is passing over the solderreservoir. This is achieved by turning off the compressed air supplywhen the membrane apparatus is used.

Whereas nitrogen has been referred to as a preferred embodiment forshield gas, other inert gases may be provided, carbon dioxide being oneexample. In all cases it is preferred to keep the oxygen content lessthan about 10% by volume.

Various changes may be made to the embodiments shown herein withoutdeparting from the scope of the present invention which is limited onlyby the following claims.

The embodiments of the present invention in which an exclusive propertyor privilege is claimed are defined as follows:
 1. A process of wavesoldering an element comprising the steps of:projecting at least onesolder wave from a solder nozzle above a solder reservoir containingsolder, through a slot in a cover means over at least a portion of thesolder reservoir; providing gas delivery means positioned on each sideof the solder wave underneath the cover means to pass shield gas upwardsthrough the slot on both sides of the solder wave and blanket the solderwave; supporting the element by conveyor means; conveying the element bythe conveyor means in a predetermined path above the cover means so thatat least a portion of the element passes through the solder wave whileblanketed by the shield gas, and directing shield gas passing upwardsthrough the slot to blanket side edges of the element during the passingof at least a portion of the element through the solder wave.
 2. Theprocess of wave soldering an element according to claim 1 wherein theelement is a printed circuit board having an under surface, a frontedge, a rear edge and opposing side edges, the front edge passingthrough the solder wave first, the solder wave contacting the undersurface of the circuit board, and the rear edge leaving the solder wavelast.
 3. The process of wave soldering an element according to claim 2wherein the conveyor means comprises a first conveyor having grippingmeans supported by a first rail at one end of the solder wave and asecond conveyor having gripping means supported by a second rail at theother end of the solder wave, the distance between the gripping means ofthe first conveyor and the gripping means of the second conveyor beingadjustable for different sizes of printed circuit boards.
 4. The processof wave soldering an element according to claim 3 wherein the grippingmeans are gripping fingers which grip the opposing side edges of thecircuit board.
 5. The process for wave soldering an element according toclaim 3 wherein the shield gas is directed by end shroud means tosubstantially uniform blanketing of the gripping means of the first andsecond conveyors and the side edges of the printed circuit board duringthe passing of at least a portion of the element through the solderwave.
 6. The process of wave soldering an element according to claim 5wherein the end shroud means includes end shroud panels fitting overeach end of the solder wave, the end shroud panels being at an elevationabove the solder wave and having sealing means resting on the covermeans and extending through a portion of the solder wave.
 7. The processof wave soldering an element according to claim 1 wherein the shield gasis substantially oxygen free.
 8. The process of wave soldering anelement according to claim 1 wherein the shield gas is primarilynitrogen.
 9. The process of wave soldering an element according to claim8 wherein the nitrogen is made by a membrane process.
 10. The process ofwave soldering an element according to claim 9 wherein the nitrogen hasan oxygen content of less than about 10% by volume.
 11. The process ofwave soldering an element according to claim 9 wherein the nitrogen hasan oxygen content of less than about 5% by volume.
 12. The process ofwave soldering an element according to claim 8 wherein the nitrogen hasan oxygen content in the range of about 1 ppm to 50,000 ppm by volume.13. The process of wave soldering an element according to claim 9wherein the nitrogen flow is in the range of about 200 to 600 cubic feetper hour.
 14. The process of wave soldering an element according toclaim 1 wherein the shield gas is primarily carbon dioxide.
 15. Theprocess of wave soldering an element according to claim 14 wherein thecarbon dioxide has an oxygen content of less than about 10% by volume.16. The process of wave soldering an element according to claim 1wherein the gas delivery means comprise gas diffusers positioned on eachside of the solder wave underneath the cover means.
 17. The process ofwave soldering an element according to claim 1 wherein the solder waveis a bi-directional non-turbulent flow solder wave.
 18. The process ofwave soldering an element according to claim 1 wherein the solder waveis vibrated.
 19. The process of wave soldering an element according toclaim 1 wherein the shield gas is supplied only when the element ispassing over the solder reservoir.
 20. The process of wave soldering anelement according to claim 2 wherein the solder wave is projected fromthe solder nozzle and the shield gas is supplied only in the presence ofa board about to pass through the solder wave.
 21. The process of wavesoldering an element according to claim 1 wherein the shield gasblankets the side edges of the element prior to, during and subsequentto the passing of at least a portion of the element through the solderwave.
 22. A process of wave soldering a printed circuit board having anunder surface, front edge, rear edge and side edges, comprising thesteps of:projecting at least one solder wave from a solder nozzle abovea solder reservoir containing solder, through a slot in a cover meansover at least a portion of the solder reservoir; providing gas deliverymeans underneath the cover means to pass shield gas upwards through theslot on both sides of the solder wave and blanket the solder wave;conveying the circuit board by conveyor means in a predetermined pathabove the cover means, the front edge of the circuit board entering thesolder wave, the circuit board passing through the solder wavecontacting the under surface of the circuit board, the rear edge of thecircuit board exiting the solder wave, while the front edge, undersurface and rear edge of the circuit board are blanketed by the shieldgas, and directing shield gas to blanket the side edges of the circuitboard passing through the solder wave.
 23. The process of wave solderinga circuit board according to claim 22 wherein the under surface, frontedge, rear edge and side edges of the circuit board are blanketed byshield gas substantially uniformly while passing through the solderwave.
 24. The process of wave soldering a circuit board according toclaim 22 wherein shield gas is directed in a direction substantiallyperpendicular to the predetermined path from ends of the solder wavetowards the side edges of the circuit board passing through the solderwave.
 25. The process of wave soldering a circuit board according toclaim 22 wherein air is substantially excluded from the side edges ofthe circuit board by end shroud means.
 26. An apparatus for wavesoldering an element comprising:a solder reservoir adapted to containmolten solder, and having at least one solder wave nozzle projectingtherefrom; pump means for forming a solder wave from the nozzle; covermeans for covering at least a portion of the reservoir having at leastone longitudinal slot for the solder wave to pass therethrough; gasdelivery means for supplying gas underneath the cover means, the gaspassing upwards through the slot on both sides of the solder wave toprovide a gas blanket over the solder wave; conveyor means forsupporting the element and moving the element in a predetermined pathover the cover means ensuring at least a portion of the element passesthrough the solder wave, and end shroud means over each end of thesolder wave to direct the gas passing upwards through the slot in thecover means to blanket side edges of the element during the passing ofat least a portion of the element through the solder wave.
 27. Theapparatus for wave soldering an element according to claim 26 whereinthe conveyor means comprises a first conveyor having gripping means, thefirst conveyor at one end of the solder wave supported by a first railand a second conveyor having gripping means, the second conveyor at theother end of the solder wave supported by a second rail, adjustmentmeans to vary the distance between the gripping means of the firstconveyor and the gripping means of the second conveyor for differentsizes of elements.
 28. The apparatus for wave soldering an elementaccording to claim 27 wherein the gripping means comprises grippingfingers to grip the side edges of the board.
 29. The apparatus for wavesoldering an element according to claim 27 wherein the first rail is afixed rail and the second rail is an adjustable rail.
 30. The apparatusfor wave soldering an element according to claim 27 wherein the endshroud means comprise a first end shroud panel resting on the covermeans beneath the fixed rail of the first conveyor and a second endshroud panel resting on the cover means connected to the adjustable railsupporting the second conveyor.
 31. The apparatus for wave soldering anelement according to claim 30 wherein the first and second end shroudpanels have silicone rubber rib seals in a U-configuration extending onboth sides of the solder wave and across the solder wave to pass througha tip of the solder wave.
 32. The apparatus for wave soldering anelement according to claim 30 including at least one extra end shroudpanel to cover an exposed end of the solder wave when the adjustablerail for the second conveyor is moved towards the fixed rail of thefirst conveyor.
 33. The apparatus for wave soldering an elementaccording to claim 26 wherein the end shroud means over each end of thesolder wave ensures substantially uniform gas blanketing of the elementends during the passing of at least a portion of the element through thesolder wave.
 34. The apparatus for wave soldering an element accordingto claim 26 including a membrane nitrogen producing means to producenitrogen for the gas delivery means.
 35. The apparatus for wavesoldering an element according to claim 26 wherein the cover meanscomprises a cover plate on each side of the solder wave extending overat least a portion of the reservoir and having skirts at four edges ofthe cover plate, the skirts substantially sealed at corners extendingdown below a level representing a solder level in the reservoir.
 36. Theapparatus for wave soldering an element according to claim 35 whereineach cover plate is supported at each side by side skirts and includingmeans for sliding each cover plate to change the width of thelongitudinal slot.
 37. The apparatus for wave soldering an elementaccording to claim 26 wherein the cover means is supported from thesolder wave nozzle.
 38. The apparatus for wave soldering an elementaccording to claim 26 wherein the gas delivery means extends on bothsides of the solder wave below the cover means.
 39. The apparatus forwave soldering an element according to claim 38 wherein the gas deliverymeans comprise gas diffusers.
 40. The apparatus for wave soldering anelement according to claim 26 including vibratory means in the solderwave nozzle to vibrate the solder wave.
 41. The apparatus for wavesoldering an element according to claim 26 wherein the pump means hasvariable adjustment for changing the configuration of the solder wavefrom the nozzle.
 42. The apparatus for wave soldering an elementaccording to claim 26 wherein at least one solder wave nozzle produces abi-directional non-turbulent solder wave.
 43. The apparatus for wavesoldering an element according to claim 26 including an on sitenon-cryogenic nitrogen producing means.
 44. The process of wavesoldering an element according to claim 8 wherein the nitrogen is madeon site by a non-cryogenic process.